Our invention relates to an overdrive protection circuit, and particularly to an overdrive circuit that permits the output power of a radio frequency amplifier to be maintained at a predetermined level, but that prevents the output power from exceeding the predetermined level by more than a predetermined amount despite changes in the load connected to the radio frequency amplifier.
Radio frequency power amplifiers have been provided with feedback circuits or levelling loops which maintain the radio frequency power output at a selected level, despite changes in the load. For example, a radio frequency amplifier may supply power to a radiating antenna which may present an impedance that varies considerably where such an antenna and amplifier are on an automobile. As the automobile moves around, it may be in an open area where there is no reflection, so that the antenna presents the proper impedance. However, in cities or other locations, the antenna may be close to a metallic body which causes considerable reflection, thus raising the impedance presented by the antenna. In the prior art feedback circuits, this increased impedance caused the feedback circuit to provide a signal that increased the power output from the amplifier. Such a condition often damaged or destroyed the amplifier and associated circuit. The prior art overcame this problem with a circuit that limited the available radio frequency drive signal. However, limiting the radio frequency drive also limits the output power that can be obtained from the radio frequency power amplifier. If, as is normal, manufacturing and component tolerances were allowed for the amplifier, some amplifiers could not produce the nominal power output needed for the transmitter.
Accordingly, a primary object of our invention is to provide a new and improved overdrive protection circuit that limits the power produced by a radio frequency power amplifier without limiting the available radio frequency drive.
Another object of our invention is to provide a new and improved overdrive protection circuit to be used with a radio frequency amplifier having a feedback circuit for maintaining the output power at a predetermined level, the overdrive protection circuit being capable of preventing the amplifier output power from exceeding the predetermined level by more than a predetermined amount, despite changes in the amplifier load.
Another object of our invention is to provide a new and improved overdrive protection circuit that limits the power produced by a radio frequency amplifier to a selected level above normal, despite the fact that the radio frequency amplifier has a feedback loop that is intended to keep the output power at a normal level and despite the fact that the radio frequency drive to the amplifier can be as large as needed to compensate for manufacturing and component tolerances.